Scale is encountered in heat exchangers used in the conditioning of sludges. As incoming sludge is heated, scale is formed because of inverse solubilities of scale forming substances. These scales are limited in unconditioned sludge because of the low concentrations or absence of appreciable sulphate, oxalate and phosphate ions. After the thermal conditioning, these materials are present in quantities above their relative solubility, so that increased scaling occurs after the reactor. These scales are considered insoluble but as a matter of fact do have a small solubility, and by reversing the flow, the unconditioned or cooler sludge passing over the scale will dissolve some of it. It is the object of the present invention to remove the scale while reducing the down time needed to clean and maintain the heat exchangers, and to improve the efficiency and performance of thermal conditioning without the use of chemicals.